oyasirazu ga arimasu.

It is usually read おやしらず.

A few notes:

• 親 = parent
• 知らず comes from 知る = to know, and here it means not knowing

So the word literally has the image of a tooth that comes in when your parents don’t know / aren’t involved anymore, which is why it means wisdom tooth.

That is very common in Japanese.

In this word:

• 親 is written in kanji
• しらず is usually written in hiragana as 知らず or しらず

You may see:

• 親知らず
• 親不知
• sometimes even all hiragana in learner-friendly materials

親知らず is a very standard and natural spelling.

が marks what exists in sentences with ある / いる.

So in 親知らずがあります, the thing that exists is 親知らず.

A very literal structure is:

• wisdom tooth/teeth + exists

That is why Japanese uses が here, even though in English we often translate it as have.

Japanese very often expresses have with an existence pattern.

So instead of saying I have a wisdom tooth, Japanese often says something closer to:

• There is a wisdom tooth
• with the owner understood from context

In everyday Japanese, this is completely normal for possessions, physical features, and body-related things.

Because 親知らず is treated as a thing, not as a living creature.

Japanese generally uses:

• あります for inanimate things
• います for animate things such as people and animals

A tooth is part of the body, but grammatically it is still treated like a thing, so あります is correct.

It is omitted.

Japanese often leaves out information that is obvious from context. If the speaker is talking about their own mouth or their dental situation, I does not need to be stated.

So 親知らずがあります can naturally mean:

• I have a wisdom tooth
• I have wisdom teeth

depending on context.

If you wanted to state it explicitly, you could say:

• 私は親知らずがあります。

But this can sound a bit less natural in many situations because the I is already obvious.

It can mean either, depending on context.

Japanese nouns usually do not change form for singular and plural, so 親知らず can refer to:

• one wisdom tooth
• wisdom teeth in general

If the exact number matters, Japanese would usually add something else, such as:

• 親知らずが一本あります = I have one wisdom tooth
• 親知らずが四本あります = I have four wisdom teeth

Yes, it can be natural, especially in context.

For example, if someone asks about your teeth, a dentist is examining you, or you are discussing dental problems, 親知らずがあります is perfectly understandable.

However, in many real conversations, Japanese often makes the location or owner clearer, for example:

• 親知らずがあります。 = I have a wisdom tooth / wisdom teeth.
• 奥歯のところに親知らずがあります。 = There is a wisdom tooth in the back of my mouth.
• 親知らずが生えています。 = My wisdom tooth is coming in / has grown in.

They are related, but not the same.

• 親知らずがあります focuses on existence: there is a wisdom tooth / I have wisdom teeth.
• 親知らずが生えています focuses on the tooth growing in / being erupted.

So:

• あります = it exists
• 生えています = it has come in, is growing, is present as an erupted tooth

If you are talking specifically about whether the tooth has emerged, 生えています is often more precise.

Yes, but it changes the nuance.

• 親知らずがあります is a neutral statement: There is a wisdom tooth / I have wisdom teeth.
• 親知らずはあります gives 親知らず topic or contrast emphasis, something like:
  • As for wisdom teeth, I do have them
  • I have wisdom teeth, at least

So が is the more basic choice here.

あります is polite, non-past form.

It is appropriate in normal conversation, with strangers, in class, with medical staff, and in many everyday situations.

Related forms:

• ある = plain form
• あります = polite form
• ありません = polite negative
• ありました = polite past

So a plain-form version would be:

• 親知らずがある。

But the sentence you were given is the polite version.